This application claims the benefit of Korean Patent Application No. 00-34505, filed Jun. 22, 2000, in the Korean Industrial Property Office, the disclosure of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a polymeric gel electrolyte and a lithium battery employing the same, and more particularly, to a polymeric gel electrolyte and a lithium battery having improved reliability and safety by using the polymeric gel electrolyte.
2. Description of the Related Art
As it has been known that ionic conductivity characteristics have improved when an ethylene oxide homopolymer and an alkali metal ion system are employed, a polymer solid electrolyte has been intensively researched. As a result of this research, much attention has been paid to polyether, such as polyethylene oxide, as a polymer matrix with respect to high movement freedom and solubility of a metallic cation.
The movement of metal ions occurs at an amorphous area of polymer rather than at a polymer crystalline area of a polymer. Thus, in order to reduce the crystallizability of polyethylene oxide, methods of copolymerizing polyethylene oxide with various epoxide compounds have been proposed (U.S. Pat. Nos. 5,968,681 and 5,837,157).
Lithium secondary batteries are classified according to the kind of electrolyte used, into lithium ion batteries and lithium ion polymer batteries. The lithium ion batteries generally use a cylindrical case or prismatic case as the case for sealing an electrode assembly. However, recently, a pouch has been in widespread use instead of cases, such as ones discussed above. The reason for using a pouch for sealing an electrode assembly instead of a case is that as the energy density per unit weight and volume increases, thin and lightweight batteries can be attained, and the material cost for sealing the electrode assembly can be reduced.
FIG. 1 is an exploded perspective view showing an example of a lithium ion battery using a pouch instead of a case.
Referring to FIG. 1, a lithium ion battery includes an electrode assembly 10 having a cathode 11, an anode 12, a separator 13, and a pouch 14 surrounding and hermetically sealing the electrode assembly 10. Here, the electrode assembly 10 is formed such that the separator 13 is interposed between and wound up with the cathode 11 and the anode 12. A cathode tap 12 and an anode tap 12xe2x80x2, serving as electrical passageways between the electrode assembly 10 and the outside, are drawn from the cathode 11 and the anode 12, respectively, to form electrode terminals 13 and 13xe2x80x2.
FIG. 2 is an exploded perspective view showing an example of a conventional lithium ion polymer battery.
Referring to FIG. 2, the lithium ion polymer battery includes an electrode assembly 21 having a cathode, an anode, a separator, and a pouch 22 surrounding and hermetically sealing the electrode assembly 21. Electrode terminals (or lead wires) 24 and 24xe2x80x2, serving as electrical passageways for inducing the current formed at the electrode assembly 21 to the outside, are connected to a cathode tap 23 and an anode tap 23xe2x80x2, which are provided at the cathode and the anode, respectively, and are exposed outside the pouch 22 by a predetermined length.
As described above, in the lithium ion battery shown in FIG. 1 and the lithium ion polymer battery shown in FIG. 2, the electrode assemblies 10 and 21 are put into the pouches 14 and 22 and an electrolytic solution is injected thereto, with the electrode terminals 13 and 13xe2x80x2 and 24 and 24xe2x80x2, respectively, being partially exposed. Then, heat and pressure are applied so that the thermally adhesive materials in the edges of upper and lower pouch parts are adhered to be sealed, thereby completing the battery.
As described above, since the electrolytic solution is injected after the electronic assemblies are put into the pouches, in the case of using an organic solvent having a low melting point, the electrode assembly or pouch may swell, resulting in deterioration in the reliability and safety of the battery.
To solve the above-described problems, there have been proposed several methods of fabricating batteries such that electrodes and electrolyte may be prepared by curing, by way of by UV rays or electron beams. Alternatively, electrode plates may be coated with gel without separately injecting an electrolytic solution, as disclosed in U.S. Pat. Nos. 5,972,539, 5,279,910, 5,437,942 and 5,340,368. In practice, swelling of an electrode assembly or pouch can be somewhat mitigated, which is, however, not yet satisfiable.
To solve the above-described problems, the present inventor has carried out various researches and completed the present invention relating to a gel electrolyte formed by using a polyethylene oxide polymer.
Accordingly, it is an object of the present invention to provide a polymeric gel electrolyte which can effectively suppress swelling of a battery due to an electrolytic solution.
It is another object of the present invention to provide a lithium battery which can prevent the reliability and safety of the battery from lowering due to swelling of the battery due to an electrolytic solution by using the polymeric gel electrolyte.
It is yet another object of the present invention to provide a method of manufacturing the lithium battery.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects, there is provided a polymeric gel electrolyte prepared by dissolving a terpolymer having a repeating unit represented by formula (1), a repeating unit represented by formula (2) and a repeating unit represented by formula (3) in an organic solvent having a low boiling point, mixing a lithium salt and organic solvent therewith to obtain a composition for forming an electrolyte, and then removing the low boiling point organic solvent from the composition: 
wherein n is an integer from 1 to 12 and R is an alkyl having 1 to 12 carbon atoms.
Preferably, the content of the repeating unit represented by formula (1) is about 0.1 to 0.9 mol, the content of the repeating unit represented by formula (2) is about 0.1 to 0.8 mol, and the content of the repeating unit represented by formula (3) is about 0.01 to 0.8 mol.
To achieve the object of providing a lithium battery using a polymeric gel electrolyte, there is provided a lithium battery including an electrode assembly having a cathode, an anode a separator interposed between the cathode and the anode, a polymeric gel electrolyte prepared by dissolving the terpolymer having a repeating unit represented by formula (1), a repeating unit represented by formula (2) and a repeating unit represented by formula (3) in a low boiling point organic solvent, mixing a lithium salt and an organic solvent therewith to obtain a composition for forming an electrolyte, injecting the composition into a case for accommodating an electrode assembly or coating the composition on at least one of the cathode, the anode and the separator, and removing only the low boiling point organic solvent from the resultant structure, and a case for accommodating the electrode assembly and polymeric electrolyte: 
wherein n is an integer from 1 to 12 and R is an alkyl having 1 to 12 carbon atoms.
To achieve the object of providing a method of manufacturing a lithium battery, there is provided a composition for forming an electrolyte by dissolving a terpolymer having a repeating unit represented by formula (1), a repeating unit represented by formula (2) and a repeating unit represented by formula (3) in a low boiling point organic solvent, and mixing a lithium salt and an organic solvent therewith, injecting the composition into a case accommodating an electrode assembly having a cathode, an anode and a separator, and removing the low boiling point organic solvent from the resultant structure to obtain a polymeric gel electrolyte to then be hermetically sealed: 
wherein n is an integer from 1 to 12 and R is an alkyl having 1 to 12 carbon atoms.
Also, to achieve the this object, there is provided a composition for forming an electrolyte by dissolving a terpolymer having a repeating unit represented by formula (1), a repeating unit represented by formula (2) and a repeating unit represented by formula (3) in a low boiling point organic solvent, and mixing a lithium salt and an organic solvent therewith;
coating the composition on at least one of a cathode, an anode and a separator; and
forming an electrode assembly using the cathode, anode and separator, removing the low boiling point organic solvent from the resultant structure to obtain a polymeric gel electrolyte to then be hermetically sealed: 
wherein n is an integer from 1 to 12 and R is an alkyl having 1 to 12 carbon atoms.
The low boiling point organic solvent includes acetone, tetrahydrofuran and the like.